Plant secondary metabolites as mammalian feeding deterrents:: separating the effects of the taste of salicin from its post-ingestive consequences in the common brushtail possum (Trichosurus vulpecula)

The effect of the phenolic glycoside, salicin, on food intake of the common brushtail possum (Trichosurus vulpecula) was studied in a series of feeding experiments. Increasing the concentration of salicin in a diet of fruits and cereals led to significant reductions of food intake in the short term (6 days). After prolonged (20 days) exposure to salicin, food intake (19 g kg(-0.75) day(-1)) was still reduced relative to controls (31 g kg(-0.75) day(-1)) but not reduced to the same extent as in the short-term experiments. Nonetheless, over these 20 days, common brushtail possums regulated their intake of salicin so as not to exceed a threshold limit of 1.9 +/- 0.1 g kg(-0.75) day(-1). Manipulative experiments sought to determine whether this threshold intake was in response to pre-ingestive factors (taste) or the post-ingestive consequences of ingesting salicin. Dietary salicin (0.17-5.0% DM) had no significant effect on nitrogen balance or urea metabolism and injection of a specific serotonin receptor antagonist, ondansetron. did not lead to increases in salicin intake as has been found for some other plant secondary metabolites. Similarly, administration of 1.3 g salicin by gavage had no significant effect on the subsequent intake of salicin compared to controls that were gavaged with water. We concluded that pre-ingestive factors were responsible for common brushtail possums limiting their intake of salicin-rich diets rather than any measurable post-ingestive consequence of feeding.